38 The Theory of Irrigation. [January, 



with gradually by evaporation and percolation. The 

 water absorbed by the roots of a large tree has been 

 found to be greatly in excess of the weight of that fluid 

 which enters into new combinations resulting in its growth, 

 and the superfluous moisture must somehow be carried off 

 almost as rapidly as it flows into the tree. " Recent 

 experiments* on this subject by Von Pettenkofer were made 

 with an (3ak tree, extending over the whole period of its 

 summer growth. The total amount of evaporation in the 

 year was estimated at 539*16 c.c. of water for the whole 

 area of its leaves. The average amount of rainfall for the 

 same period was only 65 c.c. ; and the amount of evapora- 

 tion was thus 8j times more than that of the rainfall." 

 This evaporation of the juices of the plant, by whatever 

 process effected, takes up atmospheric heat and produces 

 refrigeration, increasing, at the same time, the humidity of 

 the air by pouring out into the atmosphere, in a vaporous 

 form, the water it draws up through its roots. 



Although the destruction of forests can hardly be said to 

 influence the total amount of rainfall, it has no doubt, 

 owing to the circumstances above mentioned, no small 

 effect upon its distribution. The most obvious argument in 

 favour of this supposition is that the summer and even the 

 mean temperature of the forest is below that of the open 

 country adjoining. This must reduce the temperature of the 

 atmospheric stratum immediately above it, and, of course, 

 whenever a saturated current sweeps over it, it must produce 

 precipitation which would fall upon or near it. 



The manner in which forest destruction has most directly 

 led to the necessity for irrigation, is, perhaps, the effect 

 which it has upon the flow of springs. The roots of forest 

 trees penetrating far below the superficial soil conduct the 

 water accumulated on its surface to the lower depths to 

 which they reach, and thus serve to drain the superior strata 

 and remove the moisture out of the reach of evaporation. 

 This ensures the permanence and regularity of natural 

 springs, not only within the limits of the wood, but at some 

 distance beyond its borders, and so contributes to the 

 supply of an element essential both to vegetable and animal 

 life. As the forests are destroyed, the springs which flowed 

 from the woods, and, consequently, the greater watercourses 

 fed by them, diminish both in number and in volume. 

 Boussingault, in his " Economie Rurale," remarks that, 

 " since the clearing of the mountains in many localities, 

 the rivers and the torrents, which seemed to have lost a 



* " Quarterly Journal of Science," October, 1870, p. 524. 



